A fuel cell system transforms the energy of fuel, directly into electrical energy. In the fuel cell system disclosed by JP8-106914A published by the Japanese Patent Office in 1996, in a pair of electrodes provided on either side of a membrane electrode, fuel gas containing hydrogen is supplied to the anode and an oxidizing agent gas containing oxygen is supplied to the cathode. Electrical energy is then extracted from the electrodes, using the electrochemical reactions produced on the surface of the membrane electrode, i.e.,                anode reaction: H2→2H++2e−        cathode reaction: 2H++2e−+(½)O2→H2O        
The fuel gas supplied to the anode, may be supplied directly from a hydrogen storage device, or fuel containing hydrogen may be reformed, and the reformed hydrogen-containing gas supplied to the anode. The fuel containing hydrogen may be natural gas, methanol or gasoline, and the oxidizing agent gas supplied to the cathode is generally air.
As it is necessary to keep the humidification state of the membrane electrode optimal in order to extract the performance of the membrane electrode in the fuel cell and enhance the power generation efficiency, the fuel gas and air introduced into the fuel cell is humidified. When reforming fuel gas and extracting hydrogen as mentioned above, water is used for reforming. Therefore, in order to use the fuel cell for vehicles, a water balance must be maintained within the fuel cell system including the fuel cell or reforming device. This is because the practicality of a fuel cell vehicle will fall remarkably if water runs short and it becomes necessary to supply pure water periodically.
There are two means of supplying water to the membrane electrode. One is the method of humidifying the fuel gas or air with a humidifier, and humidifying the membrane electrode using the moisture. Another is the method of connecting a cooling water passage to the anode and cathode in the fuel cell via a bipolar plate comprising a porous material, and supplying water to the membrane electrode from the cooling water to humidify the membrane electrode. Since the latter method does not need a humidifier for fuel gas and air, there is an advantage in that the system construction is simpler.